In ink-jet printing tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver. Printing can be accomplished by moving a print head across the ink-receiver or vice versa.
The jetting of the ink droplets can be performed in several different ways. In a first type of process called continuous ink-jet printing, the ink stream jetted from an orifice of the print head is broken up, by applying a pressure wave pattern to this orifice, into ink droplets of uniform size and spacing, which can be electrostatically charged or not as desired. In one embodiment the charged drops are deflected by an electric field into a gutter for recuperation, while the uncharged drops are undeflected and land on the ink-receiver to form an image. In an alternative embodiment it is the charged droplets which land on the ink-receiver to form an image and it are the uncharged droplets, which are recuperated.
According to a second process the ink droplets can be created by a “drop on demand” method (DOD). A drop-on-demand device ejects ink droplets only when they are needed for imaging on the ink-receiver, thereby avoiding the complexity of drop charging, deflection hardware, and ink collection. In drop-on-demand ink-jet printing, the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pushes (so-called “bubble jet” method, or “thermal jet” method).
The ink fluids can be roughly divided into:                water based, the drying mechanism involving absorbance, penetration and evaporation;        oil based, the drying involving absorbance and penetration;        solvent based, the drying mechanism involving penetration but primarily evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        radiation curable, in which drying is replaced by polymerization.        
Water based, oil based and solvent based inks are jetted on ink-receivers, which typically contain either one or more porous layers that imbibe the ink via capillary action, or one or more polymer layers that swell to absorb the ink. Hot melt and radiation curable inks are usually jetted on substantially non-absorbing ink-receivers. Hot melt inks are limited to thermally stable ink-receivers, while radiation curable inks can be jetted on a wide variety of ink-receivers.
The main problem of radiation curable inks is that the image quality tends to change with the selection of the ink-receiver. In particular, the spreading of the ink droplet on the ink-receiver is highly dependent on the type of ink-receiver chosen.
One method to obtain a consistent image quality with a wide variety of ink-receivers would be to adapt the ink-jet ink set each time to the chosen ink-receiver. However, changing inks in printer and printhead is very time consuming and not really a viable solution for an industrial printing environment. Therefore, the general approach is to modify the surface chemistry either with a suitable surface layer coating or by pre-treatment, i.e. plasma, corona, flame treatment. Surface coatings can be either absorbing or non-absorbing.
The surface treatment in EP 1199181 A (TOYO INK) consists of a corona treatment or a plasma treatment.
EP 1199181 A (TOYO INK) discloses a method for ink-jet printing on a surface of a substrate of a synthetic resin, said method comprising the steps of:    1. conducting a surface treatment to said surface so as to provide the surface with a specific surface free energy of 65-72 mJ/m2     2. providing an activation energy beam curable ink having a surface tension of 25-40 mN/m    3. discharging the ink onto the surface having the specific surface free energy with an ink-jet printing device thereby forming printed portions of said ink on the surface and    4. projecting an activation energy beam onto the printed portions.
Corona discharge treatment is one of the most commonly used methods for activating a plastic surface prior to forming an adhesive bond. Corona discharges are produced from point sources, and as such produce localised energetic discharges, which are commonly known as streamers. The production of localised energetic discharges often result in a non-uniform treatment of the substrate and hence a non-uniform image quality. Corona treatment has the advantage of working well with newly manufactured substrates. However, recycled substrates may contain significant impurities or irregularities that may interfere with the treatment of the substrate.
Plasma treatment infuses the treatment zone with an inert gas that is partially ionized by the energized electrodes. While plasma treatment may provide enhanced adhesion in certain applications, a special inert gas or gas mixture and a complex control mechanism are required to realize plasma treatment. This increases the cost, complexity and maintenance of the equipment used to process the substrates. Thus, it is desirable to avoid the plasma treatment process where possible.
The other possibility for using the same ink-jet ink set on different ink-receivers is the application of a surface layer just prior to jetting the radiation curable ink.
WO 03101747 (DELAWARE CAPITAL FORMATION) discloses a method of selecting a coating to provide a surface to which inkjet ink adheres with a selectable and substantially consistent texture and gloss across the surface of a non-inkjet ready substrate comprising:                determining a desired adherence of the ink to the coating;        determining a desired finish.        
The dot quality clearly differs, in five of the six cases, on the three different substrates provided with the same coating. The nature of the ink-jet ink is not disclosed.
U.S. Pat. No. 6,720,042 (3M) discloses an article comprising:    a) a sheet having a primed surface portion; and    b) a radiation cured ink jetted image derived from an ink composition comprising at least 25 weight percent of at least one radiation curable monomer disposed on said primed surface portion;    wherein the article is durable for outdoor usage.
In U.S. Pat. No. 6,720,042 (3M), the range of suitable radiation curable inks is limited to inks which contain at least 25 weight percent of a monofunctional monomer functioning as a diluent or solvent. Also the primer thickness is critical to achieve an optimal compromise between dot gain and adhesion.
US 20030224128 A (3M) discloses a method of printing a non-aqueous ink comprising:    a) providing a substrate comprising a primed surface of thickness t1;    b) printing a non-aqueous ink on said primed surface, said ink having a theoretical dry thickness t2 and an actual dry thickness t3;    wherein t3 is greater than t2 by an amount ranging from about 25% of t1 to an amount about equal to the sum of t2 and t1.
Again the range of suitable inks in US 20030224128 A (3M) is limited. The ink-jet ink must contain an organic solvent capable of at least partially dissolving the primed layer. The application is silent on radiation curable inks, which is also the case for the solvent-based inks jetted on a primed substrate of WO 03080356 (UCB) and WO 2004003093 (UCB).
US 20040126507 A (EXXON MOBIL) discloses a substrate printed with a radiation cured inkjet printed image wherein the image is printed on a coating on the substrate and wherein the coating comprises a copolymer derived from a carboxylic acid containing vinylic unsaturation. The examples fail to teach how to achieve minimal differences in drop spread with the same coating on different substrates, e.g. DIGI-LYTE Film 65 (very little spread) and LABEL-LYTE Film 70 (excessive spread).
Although surface property modification by either coating or pre-treatment techniques has been widely employed, the exact nature of the ink-media interaction is not fully understood. Attempts are typically made to correlate the print quality to measurable surface parameters such as surface energy and surface roughness, but these parameters do not fully capture the behaviour of ink droplets on various media.
Therefore, it would be desirable to have an ink-jet printing process capable of producing approximately the same dotsize with the same radiation curable ink droplet volume on a wide variety of ink receivers provided with a surface layer of a fluid and wherein the thickness of the surface layer is not critical. The radiation curable inks should not be restricted to certain specific inks having enough organic solvent or monofunctional monomers present.